This invention related generally to piezoelectric actuation and, in particular, to an improved piezoelectric force motor incorporating a stacked transducer and other features to achieve a temperature expansion coefficient that approaches zero.
While there are many types of electrical motors capable of delivering rotational force, the choices for linear translation are more limited. One option is the ball-and-screw type motor, which incorporates a threaded shaft and a metal ball with a threaded hole that mates with the threaded shaft. As the shaft turns, the metal ball rides up or down the shaft to linearly displace the metal ball and any attached mechanism. The problems with this approach include high manufacturing tolerances and mechanical wear, which can lead to unacceptable positioning accuracy and premature failure.
An alternative is linear actuators and force motors based upon the piezoelectric effect. Certain piezoelectric materials are able to convert electrical energy directly into mechanical energy. When a voltage is applied, the material will experience a strain that causes it to expand. When the voltage is removed, the strain is removed and the material contracts.
Piezoelectric materials are generally formed from ceramics, and may include a plurality of laminated layers to keep the drive voltage to a practical level while delivering significant expansion. To increase power output, however, it is often necessary to use multiple or split shafts. One example is described in U.S. Pat. No. 6,246,157, which includes a split motor shaft having with two shaft segments that are coaxially affixed to opposite sides of an expandable and contractable displacement actuator. The displacement actuator has a piezoelectric body coincident with the axis of the shaft segments such that the expansion of the piezoelectric body increases the length of the split shaft. The motor has a set of clamps that receive and close on the shaft segments to hold them in place, or open to allow axial displacement of the shaft segments. Linear motion of the split shaft motor is produced by coordinating the opening and closing the clamps with the expansion and contraction of the piezoelectric body.
Other applications may require less force, but may need to be very accurate over a particular temperature range. One example is the air-gapped tunable etalon developed for various spectroscopic environmental monitoring applications. Although the actuators used for tuning, i.e., the motors, worked exceptionally well, they were not designed to be produced as a commercial product. Generally speaking, these previous designs required too much manual labor and craftsmanship to produce them in an appreciable quantity.
The previous motors were also optically contacted to the etalon plates, such that when the length of the motors was changed, a bending moment was exerted on the etalon plate, resulting in an over-constrained system. This limited the versatility that could be achieved with the approach, since etalon defect finesse is usually the limiting factor in achieving high-finesse in a Fabry-Perot etalon.
The first generation of motors also had a limited dynamic range, since they used single-element piezoelectric elements. The nominal capacitance in the feedback system also varied greatly, requiring the controllers to be matched to the motors. This, in turn, caused a variation in the sensitivity from motor to motor, which impacted the quality of the product. Such motors, like those commercially available from other vendors, are not capable of sustaining a bending moment. Any application of a bending moment results in failure of the piezoelectric element which permanently damages the motor. This requires that the assembly into which the motors will be integrated to either be designed such that bending moments cannot be applied, or it must be machined with sufficiently high precision so that when it is assembled, bending is eliminated. Both of these remedies result in a mechanism that is more complex and expensive than what would be required if the motor were capable of enduring a bending load.
Thus while this and other relatively low-volume applications may be served with engineering-intensive designs, there is an outstanding need for a piezoelectric force motor that will be competitive with existing commercial products not only in terms of price, but also in terms of robustness and dynamic range, particularly if such applications lead to a need for higher volumes.
This invention resides in an improved, temperature-compensated piezoelectric force motor featuring greater dynamic range and robustness as compared to previous motor designs. By implementing positive and negative expanding elements, the overall motor length can be held constant over temperature, thereby achieving a temperature expansion coefficient that approaches zero.
The inventive device is based upon a stacked PZT transducer wherein a central structural member is stretched to increase its rigidity and robustness. This central stretching removes the PZT element from the load path of the motor when the motor is relaxed, thereby preventing damage to the element during assembly (and during launch with respect to space-borne applications). When the piezoelectric element is powered, this central structural member also improves the failure strength of the assembly to further increase the robustness of the motor design.
The invention also has the advantage that the capacitance of the feedback system can be changed after the motor has been assembled. This allows the nominal capacitance to be optimized and standardized for the nominal controller capacitance. This improves the manufacturability in that this is no longer a single point failure mechanism in the assembly.
The invention finds applicability in various commercial products including, but not limited to, scientific etalons, nanopositioning systems, custom fiber optic assemblies, and custom CCD detectors.